The present invention relates to a switching power supply that facilitates obtaining a stable DC output from a DC power supply. Specifically, the present invention relates to a technique that facilitates improving the conversion efficiency of a resonance-type power supply.
For the countermeasures against global warming, it has been required recently to reduce the electric power consumption of electrical equipments. Especially, it has been an important problem to improve the conversion efficiencies of switching power supply apparatuses incorporated in almost-all the electrical equipments.
Among switching power supply apparatuses, various resonance-type power supply apparatuses have been proposed, since the resonance-type power supply apparatuses are advantageous for improving the conversion efficiencies thereof and for reducing electromagnetic interference (herein after referred to as “EMI”) (cf. the following Patent Documents 1 and 2).
FIG. 9 is a block diagram showing a circuit configuration similar to the circuit configuration of a conventional switching power supply described in the following Patent Document 1. The operations of the switching power supply shown in FIG. 9 will be described with reference to the wave chart described in FIG. 10.
Shown in FIG. 9 are DC power supply Vin; switching devices Q1 and Q2 such as MOSFET's; insulation transformer T1; leakage inductance Lr; exciting inductance Lm; primary winding Np1; secondary windings Ns1 and Ns2; inductor Lz; capacitors Cr and Co; rectifying diodes D1 and D2; load Ro; control circuit Cont; error amplifying circuit E/A; resistors R1 through R3; shunt regulator IC1; photocoupler PC1; gate-pulse control circuit P.Cont; and oscillator circuit VCO.
Insulation transformer T1 is shown in FIG. 9 by an equivalent circuit including leakage inductance Lr, exciting inductance Lm, primary winding Np1, and secondary windings Ns1 and Ns2. A series circuit including MOSFET's Q1 and Q2 is connected between the positive and negative terminals of DC power supply Vin. A series circuit including inductor Lz, primary winding Np1 of insulation transformer T1, and capacitor Cr is connected in parallel to MOSFET Q2. MOSFET's Q1 and Q2 are turned on and off alternately with a dead time Td interposed between the adjacent ON-periods of MOSFET's Q1 and Q2. By switching on and off MOSFET's Q1 and Q2 as described above, the voltages generated across secondary windings Ns1 and Ns2 in insulation transformer T1 are rectified and smoothed by diodes D1, D2 and capacitor Co to obtain a DC output Vo. Inductor Lz may be omitted by employing leakage inductance Lr of insulation transformer T1 in substitution for inductor Lz. The series circuit including inductor Lz, primary winding Np1 of insulation transformer T1, and capacitor Cr may be connected in parallel to MOSFET Q1, instead of MOSFET Q2 without any problem.
Control circuit Cont is formed of error amplifying circuit E/A and gate-pulse control circuit P.Cont that controls the gate pulses for MOSFET's Q1 and Q2. Error amplifying circuit E/A amplifies the error between the signal obtained by dividing the DC output Vo with resistors R1 and R2 and the reference voltage preset in shunt regulator IC1. Error amplifying circuit E/A insulates the amplified error through photocoupler PC1 and transmits the insulated signal to gate-pulse control circuit P.Cont. Gate-pulse control circuit P.Cont includes oscillator circuit VCO that changes the frequency thereof in response to the amplified and isolated error signal FB and gate-pulse generator circuit G.Cont that outputs gate pulses G1 and G2. In detail, gate-pulse generator circuit G.Cont receives the output from oscillator circuit VCO and outputs the gate pulses G1 and G2, having the same pulse width, alternately with the dead time Td interposed between the adjacent gate pulses G1 and G2. For controlling the output voltage Vo at a certain value, the oscillation frequency of oscillator circuit VCO is increased as the output voltage Vo exceeds the set voltage to the higher side and decreased as the output voltage Vo exceeds the set voltage to the lower side.
The following Patent Document 1 describes that the switching frequencies of MOSFET's Q1 and Q2 will be prevented from changing under any load condition including a no-load condition and a heavy load condition, if the ON-period of MOSFET's Q1 and Q2 is set to be longer than the half-wave period of the series resonance current that flows through a series resonance circuit formed of inductor Lz, leakage inductance Lr, and capacitor Cr.
In this case, there exist two periods Ta and Tb in the current wave forms IQ1 and IQ2 of MOSFET's Q1 and Q2 as shown in FIG. 10. In the period Ta, a series resonance current flows through the series resonance circuit formed of inductor Lz, leakage inductance Lr, and capacitor Cr. In the period Tb, a series resonance current flows through a series resonance circuit formed of inductor Lz, leakage inductance Lr, exciting inductance Lm, and capacitor Cr. During the turn-ON of MOSFET Q1 or Q2, turn-ON losses are not caused, since MOSFET Q1 or Q2 is turned on after the voltage across MOSFET Q1 or Q2 has become zero (since MOSFET Q1 or Q2 conducts zero-voltage turn-ON). As a result, a power supply that exhibits a high conversion efficiency is obtained easily.
More conventional switching power supply apparatuses of a resonance type are described in the following Patent Documents 3 and 4. The switching power supply described in the following Patent Document 3 needs an auxiliary switch. The switching power supply described in the following Patent Document 3 does not distinguish between the control under a light load and the control under a heavy or moderate load. The switching device control under a heavy or moderate load described in the following Patent document 4 is conducted in the same manner as the switching device control according to the invention. However, the switching device control under a light load described in the following Patent document 4 is different from the switching device control according to the invention. In detail, the switching frequencies of the switching power supply apparatuses described in the following Patent Document 3 and 4 are higher than the switching frequency of the switching power supply according to the invention. The conversion efficiencies of the switching power supply apparatuses described in the following Patent Document 3 and 4 are lower than the conversion efficiency of the switching power supply according to the invention.    Patent Document 1: Japanese Patent No. 3080128    Patent Document 2: Japanese Patent No. 2734296    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2001-314079    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2006-204044
For making the switching power supply described in the Patent Document 1 conduct zero-voltage turn-ON, it is necessary to set the energy, released from inductor Lz, leakage inductance Lr and exciting inductance Lm when MOSFET's Q1 and Q2 are turned on, to be higher than the energy stored in the output capacitance (parasitic capacitance, not shown) of MOSFET Q1 and the output capacitance (parasitic capacitance, not shown) of MOSFET Q2. Therefore, exciting inductance Lm is set around a relatively small value of several hundreds pH to increase the turn-OFF current. Since the energy stored in exciting inductance Lm is not released to the secondary side, the energy stored in exciting inductance Lm causes reactive power. The conduction losses of MOSFET's Q1 and Q2 increase more as exciting inductance Lm is set to be smaller, causing a reduced conversion efficiency.
The period Tc in the wave chart described in FIG. 10 is a period, for which any current does not flow through rectifying diodes D1 and D2. Therefore, the peak values and the effective values of the diode currents ID1 and ID2 are relatively large, causing a reduced conversion efficiency.
In view of the foregoing, it would be desirable to obviate the problems described above. It would be also desirable to improve the conversion efficiency of a switching power supply so that the reactive power caused by the exciting inductance may be reduced, the losses caused in the rectifying diode may be reduced, and the switching frequency may be prevented from increasing.
Further objects and advantages of the invention will be apparent from the following description of the invention.